1. Field of the Invention
This invention relates generally to the production of power frequency (50/60 Hz) AC power directly from high frequency alternators without inverters or power frequency changers, and more particularly, to an efficient method of modulating the alternator output into a power frequency sinusoid via field modulation using resonant circuit techniques.
2. Description of the Prior Art
The production of power frequency (50/60 Hz) AC power using high frequency alternators allows the engine speed to be independent of the output frequency, which allows the engine to operate at its most efficient or convenient speed, and due the high frequency, allows small units to generate high output power. Typical systems to accomplish this include the use of inverters to shape the rectified (DC) output of the high frequency alternator, or the use of power frequency conversion switching electronics to shape the high frequency output power without explicit rectification. Because these system require hard switching and manipulation of the full output current, they involve large amperage semiconductors and the associated costs, inefficiencies and heat dissipation requirements.
Hilgendorf, in U.S. Pat. No. 3,916,284, discloses a method for producing low (power) frequency AC directly from a high frequency alternator through manipulation of the field excitation. In this method the field is modulated at the desired AC power frequency and the modulated, rectified, high frequency output is commutated, with respect to the load, via soft switching at the zero crossing points of the AC power frequency output. This method eliminates the cost and losses involved with inverter or power frequency conversion manipulation of the full output current, while retaining the light weight and speed independence of high frequency alternators. Tests on the method proposed by Hilgendorf show that it suffers several major problems: the typical cores of high frequency alternators proposed by Hilgendorf consume inordinate amounts of power through eddy currents and core losses; the high inductance typical of alternator cores require high driving voltages in order to quickly charge and discharge the magnetic energy in the field; and, as proposed by Hilgendorf the energy exciting the field in each cycle is dissipated and wasted and must be replaced in the following cycle. Furthermore, the method of Hilgendorf does not account for the effect of residual magnetism in the alternator core, so the actual output voltage never does reduce to zero.